In an air conditioning apparatus for a vehicle which is applicable to an electric vehicle, a hybrid vehicle or the like, a heating operation cannot be performed by using combustion exhaust heat such as an engine coolant. The air conditioning apparatus can use exhaust heat from a traction motor or a battery in replacement of an engine, but since the amount of the exhaust heat is small, a heating system cannot operate with only the exhaust heat as a heat source. Even in the hybrid vehicle, an engine is controlled not to run as much as possible due to fuel saving and thus, it is intended that the hybrid vehicle adopts a heat pump heating system using a refrigerant or an electric heater heating system using a coolant as a refrigerant. Power is significantly consumed to perform a heating operation and thus, it is desirable to adopt the heat pump system that is a high COP heating system (the COP of the electric heater≤1). However, when the heat pump heating system operates at a low outside temperature (for example, −10° C.), there is a possibility that moisture in air condenses, frost formation occurs in a heat exchanger disposed on the outside of a cabin and thus, heating performance deteriorates.
For example, when frost formation occurs in an external heat exchanger for a domestic room air conditioner, a so-called reverse operation (a heating operation switched to a cooling operation) is performed, and the external heat exchanger is defrosted. However, when the heat pump system for a vehicle is defrosted in the same way, it is assumed that when a heating operation stops, a visual field cannot be ensured, that is, a safety standard cannot be met.
In this point, PTL 1 discloses a circuit in which when the external heat exchanger frosts in a low ambient air temperature range, the external heat exchanger and an internal heat exchanger are disposed in parallel with each other, a part of a heating gas discharged from a compressor detours to the external heat exchanger (evaporator), the refrigerant is allowed to flow into the internal heat exchanger as well and thus, a heating operation is continuously performed together with a defrosting operation. In this circuit, a refrigerant heater compensates for deficient heat that cannot be absorbed by the external heat exchanger (the evaporator) and thus, necessary heating performance is ensured.